The present invention relates to a closed cyclone system having a flow rate distributor for the flow rate equalization, especially directed to closed cyclone systems of fluid catalytic cracking units (FCC). More specifically, the present invention relates to a distributor placed in the interconnecting section of the cyclones of the disengager (separator) vessel of the gas-solid suspension in fluid catalytic cracking processes, which distributes the gases from the cyclones of the reaction zone and the disengager vessel, so as to promote a desirable by and uniform flow rate of said gases throughout said area. The distributor further absorbs the relative movements among the cyclones caused by the differential thermal expansion.
The fluid cracking catalytic process (FCC) is the backbone of many refineries, since most of the products are highly valuable.
Therefore, for the convenient working of a refinery, it is highly desirable that the FCC units operate without interruption during long periods of time and process various kinds of feeds, including heavy feeds having boiling points above 570xc2x0 C.
Broadly, the fluid cracking catalytic process (FCC) aims at converting high boiling point hydrocarbons into light hydrocarbon fractions such as gasoline and liquid petroleum gas (LPG). Most of the gasoline included in the gasoline pool of a refinery originates generally from the FCC unit.
As a catalytic process, FCC uses catalysts that are responsible for the optimized yields in valuable products of high commercial value, such as gasoline, diesel oil and LPG, while the yields in less desirable products such as coke and fuel gas are minimized.
The friction strength features of such catalysts are as important as their chemical features. The reason of the concern with the friction strength of the FCC catalysts is easily understood in view of the high cost of the catalyst as well as of the increasing requirements in terms of the control of the particulate emissions by the refineries.
In spite of all the concern as well as the high technology involved, even so the FCC catalyst undergoes abrasion due to the high circulation velocities of the catalyst in the units. The abrasion of the FCC catalyst results in the reduction of the catalyst particle sizexe2x80x94initially microspheres having a diameter around 70 micra, towards smaller size ranges, below 20 micra.
The state-of-the-art system usually employed for the separation (disengagement) of the catalyst from the gaseous products of the FCC unit is that of cyclone disengagers, or simply cyclones.
The separation of gas-solids mixtures using cyclones is one of the oldest industrial systems and is based on the common principle of the use of centrifuge force to separate solid particles from gases. In spite of the apparent simplicity, much is still studied and discussed in the search of a better cyclone performance.
Being at first apparatuses used aiming solely to separate and recover the catalyst from the products of the unit, with time and due to the various feeds and process modifications, the FCC cyclones acquired a new perspective, intervening even in the conversion results of the unit
In order to better evaluate the above statement, it is necessary to understand the basics of the FCC process.
In the fluid catalytic cracking unit the pre-heated catalyst is thoroughly admixed to the atomized feed stock of the unit and the so-formed mixture is conveyed to the reaction zone of the unit where occur the cracking reactions. The reaction zone is generally an elongated vertical tube, which may have the mixture flow in the upward direction (riser), or downward (downer).
The cracking products, hydrocarbon gases and vapors form a gas-solid suspension with the catalyst. Such suspension should be quickly and efficiently separated on exiting the reaction zone in order to minimize the overcracking reactions, which lead to the formation of less desirable products (fuel gas and coke). Such separation is achieved through the use of cyclones placed within the disengager vessel.
In order to improve the separation of the solid catalyst particles sets of cyclones (first and second stages) are installed.
The first cyclone stage carries most of the separation job and usually recovers around 99% of the total solids fed. The first cyclone stage has the easiest job since the catalyst particles have an average particle size between 60 and 70 micra, there is a huge amount of available gas to provide the centrifugal forces and there are no problems for discharging significant amounts of gas through the cyclone leg.
The second cyclone stage receives approximately the same amount of gas as the first cyclone stage, but the solids content is several orders of magnitude less. Due to such adverse conditions as well as to the fact that the particles that reach the second stage are of a smaller average size, the typical-recovery of a second stage cyclone is of from 95 to 98% of the total solids fed.
Once the catalyst has been separated from the gases and cracking reaction products, the stripping is started, whereby the hydrocarbons adsorbed on the catalyst or entrained by the flow are removed by entrainment with stripping steam.
After the stripping section the catalyst is conveyed to the regenerator, where the combustion reactions for the removal of the coke deposited on the catalyst occur. In the interior of the regenerator, the flow rates of combustion gases may also be considerable so in order to avoid loss of catalyst entrained by the gas produced in the burning reactions, cyclones are also installed for the separation of the gas-solid mixture.
Normally in the regenerators are also installed sets of cyclones of different and consecutive stages, as in the case of the disengager vessel.
Catalyst make-up also occurs in the regenerator through the addition of amounts of virgin catalyst calculated to keep the catalyst inventory as well as the FCC unit conversion.
In the disengager as well as in the regenerator, the adequate working of the cyclones is paramount to secure a suitable performance of the unit, with at the same time minimum catalyst loss and thus minimum particulate emission.
With passing of time as well as in view of the modifications introduced in the FCC units as a function mainly of the composition of the feed stocks, it was found that there was a requirement of an ever smaller residence time for the feed and of the reaction products with the catalyst in order to minimize the overcracking reactions.
Various methods and procedures have been proposed aiming at reaching these objectives.
One of the well-known and utilized modes aiming at this objective is the so-called xe2x80x9cclosed cyclonesxe2x80x9d, which is based on the concept of the reaction zone directly connected to the cyclone disengager.
According to the concept of xe2x80x9cclosed cyclonesxe2x80x9d, the cyclones installed in is the interior of the disengager vessel are directly linked to the reaction zone of the unit; a second cyclone stage is linked to the cyclones of the reaction zone, in series, by means of a linking pipe made up of cylinders of different diameters and mounted in telescopic form to absorb the movements due to differential thermal expansions between these cyclone stages.
This concept allows to minimize the presence of particulate matter in the exiting gases from the disengager vessel and the reaction products are more rapidly separated from the catalyst so that the overcracking reactions are significantly reduced by the reduction of the contact time between the catalyst and the cracking products.
U.S. Pat. No. 4,502,947 corresponding to Brazilian patent PI 8404451 teaches the efficient and quicker separation of the products obtained by the use of cyclone disengagers directly connected to the bottom of the riser, those being linked to cyclone sets of first and second stages. Concentric pipes, mounted in telescopic form to absorb movements due to the differential thermal expansions between the riser cyclones and these cyclone stages make the link between the exit of the riser cyclone and the inlet nozzle of the first stage of the cyclone pairs. Purge and stripping vapors flow through the annular space existing between the concentric pipes together with some entrained catalyst The use of different kinds of fillings in the interior of the annular section is suggested for mounting the concentric pipes, so as to keep some room for the circulation of the stripping steam.
U.S. Pat. No. 5,569,435 corresponding to Brazilian PI 9303773 of the Applicant and herein fully incorporated as reference, teaches that a diplegless cyclone disengager also known as pseudocyclone, directly coupled to the bottom of the riser and connected to a cyclone set by means of a pipe made up of concentric cylinders, mounted in telescopic form, allows a more efficient separation of the gas-solid suspension in the disengager vessel of the FCC unit. The annular space between the concentric cylinders is dimensioned to absorb the gases from the stripping section and the purge steam of the disengager vessel, without the need of some kind of filling for the annular section.
Thus, the use of closed cyclone systems in FCC units makes possible to optimize the separation of the gas-solid suspension, thus minimizing the catalyst losses by entrainment, reducing the contact time between the catalyst and the cracking products, and avoiding the overcracking and the increase in less desirable products such as coke and gas, with obvious benefits for the refiner as well as for the environment.
However, for the efficient operation in a regime of closed cyclones in FCC units, the coupling between the cyclones of different and consecutive cyclones by using concentric pipes of different diameters mounted in telescopic form, still results in some unsolved operation difficulties.
For example, for FCC converters where the set of riser-cyclones or the set of downer-cyclones is of asymmetric construction there is the risk of non-uniform operation between the sets of cyclones due to the difference in charge loss.
Under such condition, the need to accommodate the differential thermal expansion between the cyclones of the reaction zone and the stage connected to it in the radial direction of the disengager vessel may imply in the adoption of an annular space of specific area higher than desirable to accommodate the flow rates required in the dimensioning. In this case, higher injection of purge steam in the disengager vessel would be mandatory in order to avoid catalyst entrainment from the cyclones of the reaction zone, reducing the separation efficiency and affecting the quality of the products, with important losses for the refiner as well as for the environment.
The larger demand for purge steam to the FCC unit may bring to the refiner a series of restrictions, the operation being already carried out with the steam boilers working at the upper capacity limit.
A further complex situation occurs in an unit operating with various asymmetric cyclone sets at the disengager vessel, when the asymmetry of the sets causes a differentiated flow rate among them, this unavoidably resulting in a non-optimized operation of the cyclone sets.
Thus, in spite of the trials and proposals of the specialized literature, there is not yet a solution to the flow rate equalization in closed cyclone systems that would reduce the required flow rate of the purge steam having origin in the disengager vessel, towards the interior of the cyclone connecting pipe and which would allow the uniform operation of the cyclones in FCC converters having an asymmetric construction of the riser-cyclones set or of the downer-cyclones set such as the novel solution described and claimed in the present invention.
Broadly, the present invention comprises a closed cyclone system having a distributor for the equalization of the flow rate of the cyclones of different and consecutive stages, the distributor uniformly dispensing the gases from the majority or from all the first stage cyclones.
The present invention provides the installation in the section known as the connecting pipes section having telescopic joints, of a distributor which uniformly distributes, the gases from the first stage cyclones without structural harm for the accommodation of the differential thermal expansions.
The present invention provides thus a distributor for the homogenization of the flow rates from each first stage cyclone, making the operation of the second stage cyclones uniform and minimizing the use of extra purge steam by reducing the number of telescopic joints and of the total annular specific area in the region of the cyclone connecting pipe, keeping the same annular gap and thus reducing the required flow rate for the purge steam from the disengager vessel.
The present invention provides further a distributor for the equalization of the cyclone flow rate in closed cyclone systems which allows for a homogeneous accommodation of the gas-solid mixture in the pipe, allowing a uniform operation of the cyclones in the next stage.
The present invention provides further a closed cyclone system having a distributor for the flow rate equalizations causing that the disengager vessel contains a lower amount of material in its interior as well as easier maintenance of the cyclone system.